[PATCH 4/2] sched: task_sched_runtime introduce micro optimization

[KOSAKI Motohiro <kosaki.motohiro@gmail.com>]

rq lock in task_sched_runtime() is necessary for two reasons. 1)
accessing se.sum_exec_runtime is inatomic on 32bit and 2)
do_task_delta_exec() require it.

And then, 64bit can avoid holds rq lock when add_delta is false.

Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
---
 kernel/sched/core.c |    6 ++++++
 1 files changed, 6 insertions(+), 0 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index b817e6d..24ba1c6 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2657,6 +2657,12 @@ unsigned long long task_sched_runtime(struct task_struct *p, bool add_delta)
 	struct rq *rq;
 	u64 ns = 0;
 
+	/* Micro optimization. */
+#ifdef CONFIG_64BIT
+	if (!add_delta)
+		return p->se.sum_exec_runtime;
+#endif
+
 	rq = task_rq_lock(p, &flags);
 	ns = p->se.sum_exec_runtime;
 	if (add_delta)
-- 
1.7.1